Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 419	$ 143	$ 608	$ 430
Depreciation, depletion, and amortization	158	148	313	295
Equity (loss) income	23	(27)	14	(101)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,365	3,083	6,363	6,174
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	460	401	859	825
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,905	2,682	5,504	5,349
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	186	33	325	136
Depreciation, depletion, and amortization	90	80	177	157
Equity (loss) income	2	(11)	(9)	(28)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,467	1,291	2,823	2,569
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	457	397	852	818
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,010	894	1,971	1,751
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	233	110	283	294
Depreciation, depletion, and amortization	68	68	136	138
Equity (loss) income	21	(16)	23	(73)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,898	1,792	3,540	3,605
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	3	4	7	7
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,895	$ 1,788	$ 3,533	$ 3,598